Structurally, polymers are classified as either linear or branched wherein the term "branched" generally means that the individual molecular units of the branches are discrete from the polymer backbone, yet may have the same chemical constitution as the polymer backbone. Thus, regularly reacting side groups which are inherent in the monomeric structure and are of different chemical constitution than the polymer backbone are not considered as "branches"; that is, for example, the methyl groups pendant on a polydimethylsiloxane chain or a pendant aryl group in a polystyrene are not considered to be branches of such polymers. All descriptions of branching and backbone in the present application are consistent with this meaning.
The simplest branched polymers are the comb branched polymers wherein a linear backbone bears one or more essentially linear pendant side chains. This simple form of branching, often called comb branching, may be regular wherein the branches are distributed in non-uniform or random fashion on the polymer backbone. An example of regular comb branching is a comb branched polystyrene as described by T. Altores et al. in J. Polymer Sci., Part A, Vol. 3 4131-4151 (1965) and an example of irregular comb branching is illustrated by graft copolymers as described by Sorenson et al, Preparative Methods of Polymer Chemistry, 2nd Ed., Interscience Publishers, 213-214 (1968).
Another type of branching is exemplified by cross-linked or network polymers wherein the polymer chains are connected through the use of bifunctional compounds; e.g., polystyrene molecules bridged or crosslinked with divinylbenzene. In this type of branching, many of the individual branches are not linear in that each branch may itself contain side chains pendant from a linear chain and it is not possible to differentiate between the backbone and the branches. More importantly, in network branching, each polymer macromolecule (backbone) is cross-linked at two or more sites to other polymer macromolecules. Also the chemical constitution of the cross-linkages may vary from that of the polymer macromolecules. In this cross-linked or network branched polymer, the various branches or cross-linkages may be structurally similar (called regular cross-linked) or they may be structurally dissimilar (called irregularly cross-linked). An example of regular cross-linked polymers is a ladder-type poly(phenylsisesquinone) [sic] (poly-(phenylsilsesquioxane)). Sogah et al, in the background of U.S. Pat. No. 4,544,724, discusses some of these types of polymers and gives a short review of the many publications and disclosures regarding them. U.S. Pat. No. 4,435,548, discusses branched polyamidoamines; U.S. Pat. Nos. 4,507,466, 4,558,120, 4,568,737, 4,587,329, 4,713,975, 4,871,779, and 4,631,337 discuss the preparation and use of dense star polymers, and U.S. Pat. Nos. 4,737,550 and 4,857,599 discuss bridged and other modified dense star polymers.
Other structural configurations of macromolecular materials that have been disclosed include star/comb-branched polymers, such disclosure being found in U.S. Pat. Nos. 4,599,400 and 4,690,985, and rod-shaped dendrimer polymers are disclosed in U.S. Pat. No. 4,694,064.
Hutchins et al, in U.S. Pat. Nos. 4,847,328 and 4,851,477, deal with hybrid acrylic-condensation star polymers and Joseph et al, in U.S. Pat. Nos. 4,857,615, 4,857,618, and 4,906,691, with condensed phase polymers having regularly, or irregularly, spaced polymeric branches essentially on the order of a comb structure macromolecules.
M. Gauthier et al, Macromolecules, 24, 4548-4553 (1991) discloses uniform highly branched polymers produced by stepwise anionic grafting. M. Suzuki et al, Macromolecules, 25, 7071-2 (1992) describes palladium-catalyzed ring-opening polymerization of cyclic carbamate to produce hyperbranched dendritic polyamines. Macromolecules, 24, 1435-1438 (1991) discloses comb-burst dendrimer topology derived from dendritic grafting. U.S. Pat. No. 5,041,516 discloses other dendritic macromolecules.
The various architectures of these macromolecules results in a variety of end product uses. It is desirable to produce macromolecules that are hyperbranched (containing 2 or more generations of branching) so as to enable the production of highly functional macromolecules. Increasing the functionality of a macromolecule at a multiplicity of sites within the macromolecule can make it a much more useful molecule.
Dendrimers and hyperbranched polymers have received much attention recently due to their unusual structural features and properties. In the early 1950's, Flory, J. Am. Chem. Soc., 74, 2718 (1952) discussed the potential of AB.sub.2 monomers, in which A and B are different reactive groups which react with each other to form a chemical bond, for the formation of highly branched polymers. However, the formation of high molecular weight hyperbranched polymers from AB.sub.2 monomers containing one group of type A and two of type B was not accomplished until 1988 when Kim et al., Polym. Prep., 29(2), 310 (1988) and U.S. Pat. No. 4,857,630 reported the preparation of hyperbranched polyphenylene.
Numerous other hyperbranched polymers have been reported since that time by Hawker et al., J. Am. Chem. Soc., 113, 4583, (1991); Uhrich et al, Macromolecules, 25, 4583 (1994); Turner et al, Macromolecules, 27, 1611 (1994); and others. See also U.S. Pat. Nos. 5,196,502; 5,225,522; and 5,214,122. All of these hyperbranched polymers are obtained by polycondensation processes involving AB.sub.2 monomers. In general, these hyperbranched polymers have irregularly branched structures with high degrees of branching between 0.2 and 0.8.
The degree of branching DB of an AB.sub.2 hyperbranched polymer has been defined by the equation DB=(1-f) in which f is the mole fraction of AB.sub.2 monomer units in which only one of the two B groups has reacted with an A group.
In contrast to hyperbranched polymers, regular dendrimers are regularly branched, macromolecules with a branch point at each repeat unit. Unlike hyperbranched polymers that are obtained via a polymerization reaction, most regular dendrimers are obtained by a series of stepwise coupling and activation steps. Examples of dendrimers include the polyamidoamide (PAMAM) Starburst.TM. dendrimers of Tomalia et al, Polym. J., 17, 117 (1985) or the convergent dendrimers of Hawker et al, J. Am. Chem. Soc., 112, 7638 (1990).
Recently, some highly branched polymers have been prepared in multistep processes involving a graft on graft technique that leads to a dramatic increase in molecular weight as a result of successive stepwise grafting steps. Examples of such polymers are the Combburst.TM. polymers of Tomalia et al., Macromolecules, 24, 1435 (1991); U.S. Pat. No. 4,694,064; and the "arborescent" polymers of Gauthier et al., Macromolecules, 24, 4548 (1991) and Macromolecular Symposia, 77, 43 (1994).
The preparation of hyperbranched polymers by a chain growth vinyl polymerization has not been accomplished previously.